Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
A vehicle that uses one or more battery systems for supporting propulsion, start stop, and/or regenerative braking functions can be referred to as an xEV, where the term “xEV” is defined herein to include all of the below described electrical vehicles, or any variations or combinations thereof.
A “start-stop vehicle” is defined as a vehicle that can disable the combustion engine when the vehicle is stopped and utilize a battery (energy storage) system to continue powering electrical consumers onboard the vehicle, including the entertainment system, navigation, lights, or other electronics, as well as to restart the engine when propulsion is desired. A lack of brake regeneration or electrical propulsion distinguishes a “start-stop vehicle” from other forms of xEVs.
As will be appreciated by those skilled in the art, hybrid electric vehicles (HEVs) combine an internal combustion engine (ICE) propulsion system and a battery-powered electric propulsion system, such as 48 volt, 130 volt, or 300 volt systems. The term HEV may include any variation of a hybrid electric vehicle, in which features such as brake regeneration, electrical propulsion, and stop-start are included.
A specific type of xEV is a micro-hybrid vehicle (“mHEV” or “micro-HEV”). Micro-HEV vehicles typically operate at low voltage, which is defined to be under 60V. Micro-HEV vehicles typically provide start stop, and distinguish themselves from “start-stop vehicles” through their use of brake regeneration. The brake regeneration power can typically range from 2 kW to 12 kW at peak, although other values can occur as well. A micro-HEV vehicle can also provide some degree of electrical propulsion to the vehicle. If available, the amount of propulsion will not typically be sufficient to provide full motive force to the vehicle.
Full hybrid systems (FHEVs) and Mild hybrid systems (Mild-HEVs) may provide motive and other electrical power to the vehicle using one or more electric motors, using only an ICE, or using both. FHEVs are typically high-voltage (>60V), and are usually between 200V and 400V. Mild-HEVs typically operate between 60V and 200V. Depending on the size of the vehicle, a Mild-HEV can provide between 10-20 kW of brake regeneration or propulsion, while a FHEV provides 15-100 kW. The Mild-HEV system may also apply some level of power assist, during acceleration for example, to supplement the ICE, while the FHEV can often use the electrical motor as the sole source of propulsion for short periods, and in general uses the electrical motor as a more significant source of propulsion than does a Mild-HEV.
In addition, a plug-in electric vehicle (PEV) is any vehicle that can be charged from an external source of electricity, such as wall sockets, and the energy stored in the rechargeable battery packs drives or contributes to drive the wheels. PEVs are a subcategory of xEV that include all-electric or battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicle conversions of hybrid electric vehicles and conventional ICE vehicles. BEVs are driven entirely by electric power and lack an internal combustion engine. PHEVs have an internal combustion engine and a source of electric motive power, with the electric motive power capable of providing all or nearly all of the vehicle's propulsion needs. PHEVs can utilize one or more of a pure electric mode (“EV mode”), a pure internal combustion mode, and a hybrid mode.
xEVs as described above may provide a number of advantages as compared to more traditional gas-powered vehicles using only ICEs and traditional electrical systems, which are typically 12 volt systems powered by a lead acid battery. For example, xEVs may produce fewer undesirable emission products and may exhibit greater fuel efficiency as compared to traditional vehicles and, in some cases, such xEVs may eliminate the use of gasoline entirely, as is the case of certain types of BEVs.
As xEV technology continues to evolve, there is a need to provide improved power sources (e.g., battery systems or modules) for such vehicles. For example, it is desirable to increase the distance that such vehicles may travel without the need to recharge the batteries. Additionally, it may also be desirable to improve the performance of such batteries and to reduce the cost associated with the battery systems.
Conventional xEVs have been found to be functionally limited by their electric energy systems that supply power to their electric motor/generator and vehicle accessories. Typically, an electric motor is powered by an energy source that needs to store energy suitable for high-power discharges as well as for electric demands generated by various driving conditions.
Vehicle batteries need be carefully managed to facilitate proper, stable operation. Various characteristics of the battery may be measured, including the temperature, voltage, current, and others. Other states must be estimated from the measurable characteristics, such as the total capacity and the “state of charge” (SOC), which is the measure of the amount of energy available in a battery. For xEVs, battery characteristics are sensitive to the conditions under which they are measured. It can therefore be difficult to both use and characterize the battery at the same time. In electric or hybrid vehicles, the SOC allows for an estimate of the potential distance that may be travelled by the vehicle.
Estimates of SOC are made frequently as part of the typical vehicle operation. Many means of estimating SOC exist, but most methods use some combination of two primary methods: voltage lookup and current integration. The voltage lookup method utilizes the fact that a battery's open-circuit voltage changes with the state of charge. This method would utilize a transfer function to allow a measured voltage to be used as a means of determining the SOC. The current integration method starts with a known SOC and total capacity of the battery, and adds/subtracts the electrical charge throughput from the battery.
Each of these methods has significant disadvantages. The voltage lookup method suffers from the fact that the battery's terminal voltage is only equal to the open-circuit voltage when the current is zero and the battery is completely equilibrated—something that rarely happens in practice. Further, for some battery chemistries the open-circuit voltage is nearly constant over a wide range of SOC values, making the voltage a poor predictor of SOC. The current integration suffers from the fact that accuracy degrades gradually due to measurement error and bias and non-unity battery coulombic efficiencies.
State estimation, and in particular SOC estimation, is important in the utilization of most forms of lithium-ion batteries. This is true because the battery can be damaged, or become unstable, if operated outside proscribed operating conditions. Further, lithium-ion lacks a “redox shuttle” or “top-charge” reaction that is found for nickel-metal and lead-acid chemistries, in which overcharging the battery (within limits) creates harmless byproducts. Therefore, safe operation requires the SOC be known with some reasonable level of precision.
Typically, reliable state estimation of an energy storage device, such as SOC estimation, can only be made infrequently. For example, SOC estimation based on open-circuit voltage can only occasionally be performed, and only when the vehicle is not being operated. As such, any estimate of the SOC of the energy storage device increases in inaccuracy during vehicle operation, as the time since the last accurate measurement increases. This effect is especially true of micro-hybrid, mild-hybrid, and hybrid-electric vehicles, due to the relative heavy charge/discharge duty cycles.
To overcome these inaccurate SOC estimations, conventional approaches simply provide a wide margin for operational error. Providing a large margin for operational error can significantly increase the size of the battery, increasing the cost of the vehicle overall. Reducing the error margin without a corresponding increase in accuracy can lead to overcharges and undercharges of the energy storage device, which may cause instability, failure and/or shortening of the life of the energy storage device.
Therefore, there is need for a system and method that enable accurate estimations of a state of an energy storage device used in vehicle batteries, particularly those (such as micro hybrid, mild hybrid, and hybrid-electric vehicles that utilize high-power brake regeneration and electric propulsion.